转型，

转型

class Animal {
	public void eat(){
		System.out.println("animal eatting...");
	}
}
class Bird extends Animal{
	public void eat(){
		System.out.println("bird eatting...");
	}
	public void fly(){
		System.out.println("bird flying...");
	}
}

public class TestZhuanXing {
	public static void main(String[] args) {
		Animal a = new Bird(); //向上转型
		a.eat(); //bird eatting...
		//! error: a.fly(); b虽指向子类对象，但此时丢失fly()方法
		Bird b = (Bird)a;
		b.fly();//bird flying...
		b.eat();//bird eatting...
		
	}
}

1要有继承

2要有重写

3父类引用指向子类对象